Conventional rotary lawn mowers, in which a motor-driven blade rotates about a generally vertical axis, require a guard around the blade to prevent injury to users or other people in the proximity of the mower during operation. An inevitable consequence of such a guard is the build-up of debris, principally in the form of grass cuttings, on the underside of, and around the cutter guard. Generally, such lawn mowers have large motor which provide a surplus of power, which allows the continued operation of blade in spite of the debris build-up.
In a battery-operated robotic mower, however, such a build-up of grass cuttings around the inside of the cutter guard is more critical. The use of batteries means that power needs to be conserved as much as possible to maximise the length of time for which the robotic mower can operate. This typically involves the use of smaller, lighter and usually lower-powered motors. A preferred cutting system for such applications (see patent application EP0808096) is one in which the motor drives a solid circular cutter disc of substantially smooth form at the outer edge of which are mounted a number of light weight sharpened thin steel cutter blades. These blades can rotate freely with respect to the cutter disc for minimal drag while still cutting the grass efficiently due to their inertia and sharpness. The lower power supplied to the cutting blades means that any debris build-up is likely to have a considerable detrimental effect on the operation of the mower. It will be appreciated that the practical use of a robotic lawnmower will involve cutting the grass more frequently such that often only a small amount of growth has occurred since the last cut. Such small grass elements are especially prone to sticking to and building up on surfaces with which they may come into contact.
A robotic mower typically has an outer shell, which acts as or provides support for a collision detector. This shell can also act as a guard for the blade mechanism, to a certain extent. A full guard for the blade is therefore unnecessary to the same degree as for a conventional mower.